This invention relates to a cover and a method for making said cover for hermetically sealing packages for semiconductor devices.
Many packages for semiconductor devices consist of a metal housing and a cover sealed to the housing by means of a preform fused to housing and cover. The housing and cover are often made of iron-nickel or iron-nickel-cobalt alloys or KOVAR which are gold plated. The preform usually consists of a relatively low-melting solder alloy such as gold-tin, lead-indium-silver, lead-tin-silver, lead-tin, or tin-silver alloys. Such packages, covers and alloys are disclosed in U.S. Pat. Nos. 3,340,602, 3,367,756, 3,648,357, 3,823,468, 3,874,549, 3,946,190, 4,025,716, 4,328,921 and 4,331,253.
The package covers and methods for making covers disclosed in the prior art have a number of disadvantages. The bonding between the substrate of the cover and the preform may be incomplete, or discontinuous and often gives only 60% attachment. The bonding is generally carried out above the melting temperature of the preform alloy which tends to cause a deformation of the preform resulting in an irregular surface. An irregular surface, in turn, leads to difficulties when the cover is sealed to the package housing. It would, therefore, be an improvement if covers could be produced without the above-mentioned disadvantages. Such improvement could be realized when temperatures used in bonding of the preform to the substrate can be kept below the melting point of the preform. This has been partly realized as disclosed in U.S. Pat. No. 4,328,921. According to this disclosure, the gold-tin or lead-indium-silver alloy preform is only melted partly while being attached to the substrate. This method, however, still imparts an undesirable degree of deformation. According to U.S. Pat. No. 4,332,341, preforms made from alloys of tin, lead and silver are bonded to device contacts of a titanium-palladium-gold alloy by heating the preform to a temperature below the melting point of the solder while sufficient force is applied to produce a bond while the solid is kept in a solid phase. According to U.S. Pat. No. 3,935,986, certain metals such as copper and silver are bonded by arranging an interliner between a first and a second layer, the interliner having a melting point between those of the layers and being heat-diffusible to form an alloy with at least one of the layers, pressure bonding interliner and layers to form a composite and post bonding heating the composite above the melting point of the interliner and below that of the alloy for sufficient time to initiate diffusion and solidify the new alloy. This method has the disadvantages of a two-step process, i.e., first pressure bonding the layers and then heating the composite, and of forming a new alloy. These patents do not disclose the bonding of preforms to covers for semiconductor packages. It has also been suggested in U.S. Pat. No. 3,367,756 that gold-tin alloy foil be bonded to a gold-clad KOVAR substrate by rolling at 260.degree. C. but the foil must first be leached in order to establish a good bond. It is stated in this disclosure that experiments with pressure bonding with and without heating were unsuccessful.